1. Field of the Invention
The present invention relates to a surface wave device utilizing Love waves, and more particularly, it relates to a surface wave device having a three-layer structure which is formed by a piezoelectric thin film of ZnO, Ta.sub.2 O.sub.5 or the like, electrodes, and a piezoelectric single-crystalline substrate.
2. Description of the Background Art
For example, a VHF or UHF portable radio communication device must be reduced in size, weight and power consumption. In particular, a local oscillator which is employed for channel selection in such a portable radio communication device must have a wide band, in order to cope with multichannel communication. Thus, a voltage controlled oscillator (VCO), which is an important part of the local oscillator, must also have a wide band, with small size, right weight and low power consumption.
In general, a surface wave resonator utilizing Rayleigh waves is widely employed as that utilizing surface acoustic waves. In such a surface wave resonator utilizing Rayleigh waves, however, it is difficult to attain a wide band due to its small electromechanical coupling factor K. To this end, a surface wave resonator utilizing Love waves is watched with interest as a device having a large electromechanical coupling factor K, which can attain a wide band.
A conventional surface wave resonator utilizing Love waves comprises an Y-cut-X-propagation LiNbO.sub.3 substrate and an interdigital transducer (hereinafter referred to as IDT), prepared from a thin film of Au, Ag or Pt, which is formed on the substrate. The IDT is formed by a pair of comb electrodes having plural electrode fingers which interdigitate with each other. According to this structure, the electrodes of a metal which has a slower sound velocity than the LiNbO.sub.3 substrate are formed on this substrate, to excite Love waves by such a two-layer structure. The electrodes, which must have a slower sound velocity than the substrate, are made of a material such as Au, Ag or Pt, as hereinabove described.
For example, when such a surface wave resonator is formed by Au electrodes or an Y-cut-X-propagation LiNbO.sub.3 substrate, its electromechanical coupling factor K has a large value of about 58% thereby attaining a wide band.
In this case, however, the cost for the surface wave resonator is increased since the metal such as Au, Ag or Pt is high-priced.
In relation to the conventional surface wave resonator utilizing Love waves having a two-layer structure of electrodes and an LiNbO.sub.3 substrate, further, it is difficult to form a device having a low frequency as the later mention. When the metal electrodes are prepared from Au and the substrate is formed by an Y-cut-X-propagation LiNbO.sub.3 substrate, for example, Love waves are not excited unless a ratio H(Au)/.lambda. is at least 0.025 assuming that H(Au) represents the thickness of each electrode and .lambda. represents the wavelength of the as-excited Love waves. Therefore, the thickness of electrodes must be increased more than h/.lambda.=0.025 in a lower frequency. In this case, however, it is impossible to prepare the electrode fingers in desired widths and to obtain a surface wave device having desired characteristics, due to side etching and penetration of an etching solution into clearances between resist layers and the electrodes.
In the conventional Love wave device employing Au electrodes, further, temperature dependence of its resonance frequency is relatively large such that its characteristics are relatively extremely varied with temperature changes.